> Firefox is indisputably the best mainstream browser available available today outside of Tor for privacy, adblocking
The thing is, while things like privacy and adblocking are important, they are passive defensive features made necessary by others; they aren't things that make you feel good, they just make you feel less bad about certain things. When an extension could customize your browser exactly the way you wanted, that was exciting; when TreeStyleTabs felt integrated into the browser, instead of taking several seconds to load and feeling bolted on, it felt like an amazing part of _Firefox_; when mouse gestures worked reliably on every page, including new tab pages and plaintext-mime pages (really, sort this out Mozilla! No reason other than bad coding for extensions to not work on text/plain pages), it felt like a part of the browser, like Firefox was super-charged.
These are the things people can get excited about, and so gets people to show them off to others and recommend it. It's hard to talk about how this nuanced privacy feature is better for X complex reason, in casual conversation; much easier to demonstrate how you're able make the browser fit you like a glove with customizations A, B, and C (and it's more psychologically natural to want to show off positive additive features, too).
> and (even with the decline of functionality) extension support
Extension support, yes, but in terms of actual extensions, it's a yes-and-no situation. The thing about this and other customization that Mozilla doesn't seem to understand is, it's not enough to have "better" extension support, it needs to be "better enough" to overcome the difference in popularity between Firefox and Chrome, and hence the difference in availability/motivation of addon developers.
I still use Firefox as my main browser, but the extension support is not "better enough" for me anymore (see issues mentioned in the first para above), and so for the first in time actual decades, after switching from IE6 to Firefox back in the day, I'm considering switching or partially switching to another browser for my daily use (and have been doing so little by little, with Brave and qutebrowser).